1. Field
Embodiments described generally relate to methods and systems for purifying a crude aniline. More particularly, the embodiments described relate to methods and systems for reducing a concentration of cyclohexanone in a crude aniline.
2. Description of the Related Art
Aromatic amines, including aniline, are precursors for the preparation of many industrial chemicals. The largest use of aniline is in the production of methylene diphenyl diisocyanate (MDI), which can be reacted with polyols to produce polyurethane. Aniline can be produced by a catalytic hydrogenation of nitrobenzene, as shown in Reaction 1.

The catalyst can include Raney nickel; palladium on charcoal or nickel; or cobalt, copper or nickel on kieselguhr. Reaction 1, however, can result in the formation of impurities. One such impurity is cyclohexanone, which is difficult to remove from aniline by physical methods. For example, aniline and cyclohexanone cannot be readily separated by distillation because of their similarity in boiling points.
The crude aniline containing the cyclohexanone can be reacted under acidic conditions to form an imine compound, or Schiff's Base, namely cyclohexylidene aniline, as shown in Reaction 2.

The aniline can then be purified by removing the higher molecular weight cyclohexylidene aniline. The reaction conditions for Reaction 2, however, are susceptible to upsets in upstream equipment and process conditions, which can result in an off-specification product.
There is a need, therefore, for improved methods and systems for purifying a crude aniline.